My invention relates to a multicellular transistor chip, by which I mean an aggregate of a multiplicity of minute transistors, referred to as cells, which are built monolithically on a single semiconductor substrate, and more specifically to such a transistor chip of both the type having emitters in the form of a mesh and bases in the form of islands, and the type having emitters in the form of islands and bases in the form of a mesh. The multicellular transistor chip according to my invention is best suited for power switching applications.
The multicellular transistor chip is itself not new. It may be classified into two broad categories, one having an emitter in the form of a mesh, with bases formed like islands in the openings of the mesh, and the other having a base in the form of a mesh, with emitters formed like islands in the openings of the mesh. Whichever the type, the multicellular transistor chip is favorable in the uniformity of current and heat distributions as each of the arrayed cells functions as transistor. The result is a fairly broad area of safe operation, which is one of the most important requirements of power transistors in general.
In the manufacture of such multicellular transistor chips, the base electrode and emitter electrode are coupled to the base and emitter regions of the semiconductor substrate through openings formed in an insulating film, usually of silicon oxide, which overlies the substrate. Each cell requires at least one opening for the connection of the base electrode and at least one other for the connection of the emitter electrode. Openings must also be formed in the insulating film for contacting the base electrode with the annular base portion surrounding the meshed or islandlike emitters. Conventionally, such openings for the connection of the base electrode to the base regions of the substrate were all of the same size.
Multicellular transistor chips of such conventional construction had a drawback, particularly in cases where they were connected to inductive loads such as transformers and motors. The cells of the prior art chips were susceptible to breakdown when turned off, as high voltages are then imposed thereon. There have therefore been long awaited the advent of a multicellular transistor chip that is less susceptible to breakdown at the time of turnoff and, in consequence, wider in safe operating area than hitherto.